Assessment of Electrical Shorting and Metal Vapor Arcing Potential of Tin Whiskers
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Tin whiskers are conductive crystal growths that form unpredictively from tin and tin alloy surfaces. The growth of tin whiskers presents a reliability concern in electronic equipment due to their potential to create electrical shorts and metal vapor arcs. Concern with tin whiskers is increasing due to the ever tightening conductor spacing in smaller electronic products and the increased use of pure tin and lead-free tin alloys. While tin whiskers present a failure risk for electronics, a tin whisker mechanical bridging between two differently electrically biased conductors doesn't guaranteed electrical shorts due to surface films on tin whisker and conductors. The voltage must exceed a threshold level in order to produce the current flow through the tin whisker. However, the influence of contact force and presence of surface contaminations on breakdown voltage of tin whiskers has not been adequately investigated. Furthermore, whisker-induced electrical shorts can initiate destructive metal vapor arcs. The potential for metal vapor arc formation is affected by several factors, including whisker geometry, bias voltage and pressure. Previous studies demonstrated metal vapor arc formation using gold- and tin-wires; however, material and geometry differences between these test articles and actual tin whiskers have not been examined. Further, a practical guide for assessing the potential for tin whisker-induced metal vapor arc formation has not been provided. This dissertation provides characteristics and assessment of tin whisker-induced electrical shorts and metal vapor arcs. The breakdown voltage of tin whisker was measured using gold- and tin-coated probes to characterize the influence of two different contact materials on breakdown voltage. As a part of this effort, the effect of contact force on breakdown voltage and its current-voltage characteristics related with the failure mode and the possibility of electrical shorting by tin whiskers were also investigated. With regards to tin whisker-induced metal vapor arc formation, the effect of whisker geometry, bias voltage and pressure was investigated. Based on the experimental evidence, a metric defined as a function of bias voltage and resistance was proposed and the logistic regression model that can assess the likelihood of tin whisker-induced metal vapor arc formation was developed.